In French we can also say "saisir", which means "grasp", to say understand figuratively. I directly thought to "grip" when I saw "begrijpen", I was thinking ot would be a good way to memorize it. I did not suspect there was a real link :D Languages are really fascinating.

PS: the spanish "entender" is from another root imo. Makes me think in French to the verb "hear" (entendre). But, something can enter your ear and exit from the other without your brain grasping it

'Ik kan niet' translates to 'I can not'. 'I don't know' would be 'Ik weet het niet' you can take out the 'het' it still makes sense and will be understood. Although I'm not sure it's grammatically correct (I was a child when I left NL so my dutch isn't perfect!)